A typical excimer laser employs a series of preionization gaps that are distributed along the sides of the main electrodes that define the lasing space and the optical axis of the laser. Discharges that take place across these gaps generate ultraviolet radiation to condition this lasing space for a uniform discharge to take place. Typically, about twenty to forty such gaps will extend along the two sides of the main electrodes, depending on the size of the laser.
The metallic pins that form these pre-ionization gaps are subject to wear. If the wear becomes excessive on the pins of a particular gap, no discharge will take place at that gap and an asymmetry will arise, i.e. some gaps producing a discharge and some not, or the gaps not firing simultaneously. Any such asymmetry leads to a lack of reproducibility in laser performance, i.e. variations between the laser pulses in respect of their energy, timing or spatial location.
In an attempt to reduce this wear and thus increase the lifetime of the pre-ionization pins before they need to be replaced, various shapes of pins have been adopted. In particular it has been common to form one pin of each gap with an enlarged spherical end. However, these expedients have had only partial success, and have increased the cost of manufacture.